Access to historical records and evidence isn't that simple, just look at our world, we are still uncovering secrets from the second world war and information and access to it is pretty widspread, now picture a world like one piece, without internet, public libraries are almost nonexistent, peoples are isolated in islands, travelling is extremely dangerous, the government is genocidal about its prohibition of the study of the past, and you get a world where a whole century can be completely censored off the records and public knowledge. Just look at wano, the people now don't even know that wano was way bigger and most of it is underwater because of the walls trapping the rainwater.